“Over the past decades, a few epidemiological studies have assessed the association between shift work and the risk of diabetes mellitus with the inconsistent results,” said Lu, a researcher at Tongji Medical College, Huazhong University of Science and Technology in Wuhan, China.

The lack of a definitive summary of previous results prompted Lu's team to assess what’s known, they write in the journal Occupational and Environmental Medicine.

About 15 million Americans are shift workers, according to the U.S. Centers for Disease Control and Prevention. And diabetes affects about 30 million Americans, or about 9 percent of the total population.

Lu and colleagues combined and re-analyzed the data from 12 previous studies that looked at the association between shift work and chances of developing diabetes. The studies included a total of 226,652 participants and 14,595 people with diabetes.

The studies were published between 1983 and 2013. Six of the studies were conducted in Japan, with two each from the U.S. and Sweden and one each from Belgium and China.

Shift work includes working nights, evenings, rotating shifts or irregular shifts – anything other than working typical daytime hours, the authors note.

Based on their analysis, the risk of diabetes was increased by 9 percent overall for shift workers, compared to people who had never been exposed to shift work.

Male shift workers had a 28 percent greater risk of developing diabetes than their female counterparts. And people who worked rotating shifts had a 42 percent greater risk of diabetes compared to non-shift workers.

It’s not known how long the participants in those studies had been shift workers, which limits the authors’ ability to interpret their results.

The new analysis doesn’t prove that shift work causes diabetes or explain how it might do so, they acknowledge.

Insulin resistance is a condition in which the body doesn’t use insulin properly to process blood sugar. It’s also sometimes called “pre-diabetes.”

In addition, previous studies show that shift work is associated with weight gain, increase in appetite and body fat, which are major risk factors for diabetes Lu and his coauthors write.

“The overall literature in this subject right now has been fairly convincing that there is in fact an association between a misalignment of circadian rhythm and risk for diabetes,” Dr. Peter Butler said.

Butler directs the Larry L. Hillblom Islet Research Center at the David Geffen School of Medicine at the University of California, Los Angeles.

Butler, who was not involved in the study, said he wasn’t surprised that the authors found rotating shifts tended to have more of an effect. “If your circadian rhythms aren’t synchronized, it’s not at all surprising that bad things would happen.”.

But, Butler said, it’s not a problem for most people and that most people on night shifts don’t get diabetes.

“Probably about 20 percent of us are vulnerable for diabetes, and what I think probably happens is the people who get diabetes in relation to shift work are the ones who were vulnerable to getting diabetes anyway,” he said.

“It’s not like if you are one of the 80 percent who’s lucky enough to not be vulnerable and you go on shift work you are now going to get diabetes - it’s more a question if you are one of the unlucky ones who are predisposed to diabetes, then shift work may nudge you over that fence,” Butler added.

He said avoiding rotating shifts might be a good idea for people who have a strong family history of diabetes. But people who are at risk and have to work rotating shifts can still reduce the likelihood that they’ll get diabetes.

“You can counter the risks for diabetes,” he said. “There are many risks that come into play and circadian misalignment is just one risk, but if you counter that by regular exercise and good diet, you’d reduce that risk very substantially.”

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